1. Field of the Invention
The present invention relates to an image display that changes read-out speeds of image signals according to a brightness of an object.
2. Description of the Related Art
A digital camera includes a charge-coupled device (CCD) as an image pickup device and an external ranging device used to control a focus of the image pickup device (e.g., an automatic focus (AF)). The focus may also be controlled using output signals directly from the CCD without using the external ranging device.
Japanese Laid-open patent application 2001-057645 (JP '645) is related to the present invention and is incorporated in its entirety. JP '645 is directed to a fine focus control method that can be executed in a low brightness environment based on an image signal so as to prevent the resolution of the image from deteriorating. JP '645 discloses a digital camera with a focus control function including a focus lens, an image pickup device and an AF processing circuit. The focus lens forms an image of an object on an image pickup display. Further, the image pickup device converts the object image formed by the focus lens into an image signal. The AF processing circuit then controls a focal point of the focus lens based on the image signal from the image pickup device. Further, the disclosed digital camera uses the image signal directly during the focus control process when the brightness of the object is adequately high. When the brightness of the object is relatively low, the camera adds an image signal corresponding to an electric charge of pixels neighboring in a vertical direction to the image signal used during the focus control process. Further, the image signal is then amplified by an automatic gain control circuit (AGC) after the electric charges are added for the low brightness environment.
Another related invention is disclosed in Japanese Laid-open patent application 2001-128056 (JP '056), which is also incorporated in its entirety. JP '056 is directed to detecting a position having an adequate brightness to execute the AF processing when a reflected light cannot be detected due to the darkness of an object and the illumination of a fill light. In JP '056, the camera includes an object brightness detecting unit, a fill light illuminating unit, an AF detection area determining unit, a focus condition calculating unit, and a lens driving unit. The object brightness detecting unit detects a brightness of a predetermined area in the object, and the fill light illuminating unit illuminates a fill light to the object when the detected object brightness is equal to or lower than a reference value. Further, the AF detection area determining unit determines an area where a signal level of the reflected light obtained through illuminating the object is equal to or higher than a predetermined value as a detection area for AF within areas where the brightness is detected by the object brightness detecting unit. The focus condition calculating unit calculates a focus condition based on an output from the detection area for the determined AF, and the lens driving unit drives the lens based on an output from the focus condition calculating unit.
Generally, the sensitivity of the external ranging device to light is higher than that of the CCD. Accordingly, although it is impossible for a user to accurately see what is reflected as an object image in a monitor because of darkness when the storage time of the CCD/30 is driven at usual speed of 1/30 seconds, an output signal of the external ranging device may still be used to calculate a suitable auto focus. In other words, while the AF operation is executed, the monitor screen may be substantially dark.
When the AF process is executed without using the external ranging device, the range of the AF and brightness of a display are substantially the same. Further, because the sensitivity of the CCD is lower than that of the user, the user cannot adequately recognize the object on the display.
To solve this problem, an extension of the storage time of the CCD is effective assuming that the sensitivity of the CCD is constant. It is, however, difficult to precisely operate AF because of an object moving or becoming blurred during the AF process using the output signals directly from the CCD, even though the storage time of the CCD is extended (e.g., 1 second) and the object can be clearly reflected in the display.